3-D flex circuit forming

ABSTRACT

A process for deforming a flex circuit using a fixture assembly including a base plate having a recess, a wrinkle reducer plate, a stiffening block and a punch. The process including heating the fixture assembly; applying a force to the punch deforming the flex circuit into the recess in the base plate; holding the punch in contact with the flex circuit while heating the fixture assembly; cooling the fixture assembly with the punch in contact with the flex circuit; subsequent to cooling the fixture assembly, removing the force of the punch; and removing the flex circuit from the fixture assembly. The flex circuit having a recessed portion resulting from deforming the flex circuit into the recess in the base plate and a flat portion surrounding the recessed portion.

BACKGROUND

The present exemplary embodiments pertain to secure module assembliesand, more particularly, to a structure, process of making and a fixtureassembly for manufacturing components of the secure module assemblies.

Many activities require secure electronic communications. To facilitatesecure electronic communications, an encryption/decryption system may beimplemented on an electronic assembly or printed circuit board assemblythat is included in equipment connected to a communications network.

Such an electronic assembly is an enticing target for malefactors sinceit may contain codes or keys to decrypt intercepted messages, or toencode fraudulent messages. To prevent this, an electronic assembly maybe mounted in an enclosure, which is then wrapped in a security sensorand encapsulated with polyurethane resin.

A security sensor may be a web or sheet of insulating material withcircuit elements, such as closely-spaced, conductive lines fabricated onit. The circuit elements are disrupted if the sensor is tom, and thetear can be sensed in order to generate an alarm signal. The alarmsignal may be conveyed to a monitor circuit in order to reveal an attackon the integrity of the assembly. The alarm signal may also trigger anerasure of encryption/decryption keys stored within the electronicassembly.

BRIEF SUMMARY

The various advantages and purposes of the exemplary embodiments asdescribed above and hereafter are achieved by providing, according to anaspect of the exemplary embodiments, a process for deforming a flexcircuit using a fixture assembly comprising a base plate having arecess, a flex circuit, a wrinkle reducer plate, a stiffening block anda punch, the process comprising: heating the fixture assembly to apredetermined temperature; applying a force to the punch causing thepunch to contact the flex circuit, and deforming the flex circuit intothe recess in the base plate; holding the punch in contact with the flexcircuit for a predetermined amount of time while heating the fixtureassembly at the predetermined temperature; cooling the fixture assemblyto a second predetermined temperature with the punch in contact with theflex circuit; subsequent to the fixture assembly reaching the secondpredetermined temperature, removing the force of the punch; and removingthe flex circuit from the fixture assembly, the flex circuit having arecessed portion resulting from deforming the flex circuit into therecess in the base plate and a flat portion surrounding the recessedportion.

According to another aspect of the exemplary embodiments, there isprovided a process for deforming a flex circuit using a fixture assemblycomprising a base plate having a recess, a wrinkle reducer plate, astiffening block and a punch, the process comprising: clamping the flexcircuit between the wrinkle reducer and the base plate wherein thewrinkle reducer has a cut-out portion aligned with the recess andwherein the wrinkle reducer has a clamp zone at an edge of the wrinklereducer in which the flex circuit is clamped and a stretch zone betweenthe clamp zone and the first cut-out portion in which the flex circuitis not clamped when the flex circuit is placed between the base plateand the wrinkle reducer; heating the fixture assembly to a predeterminedtemperature; applying a force to the punch causing the punch to contactthe flex circuit, and deforming the flex circuit into the recess in thebase plate; holding the punch in contact with the flex circuit for apredetermined amount of time while heating the fixture assembly at thepredetermined temperature; cooling the fixture assembly to a secondpredetermined temperature with the punch in contact with the flexcircuit; subsequent to the fixture assembly reaching the secondpredetermined temperature, removing the force of the punch; and removingthe flex circuit from the fixture assembly, the flex circuit having arecessed portion resulting from deforming the flex circuit into therecess in the base plate and a flat portion surrounding the recessedportion.

According to a further aspect of the exemplary embodiments, there isprovided a module assembly comprising: a circuit board; a firstplurality of electronic components mounted on a first side of thecircuit board; a first enclosure bonded to the first side of the circuitboard and enclosing the first plurality of electronic components; asecond enclosure bonded to a second side of the circuit board, thesecond enclosure comprising: a flex circuit having a recessed profile tomatch a profile of the second enclosure; and a second plurality ofelectronic components mounted on the flex circuit wherein the flexcircuit and second plurality of electronic components are enclosed bythe second enclosure.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The features of the exemplary embodiments believed to be novel and theelements characteristic of the exemplary embodiments are set forth withparticularity in the appended claims. The Figures are for illustrationpurposes only and are not drawn to scale. The exemplary embodiments,both as to organization and method of operation, may best be understoodby reference to the detailed description which follows taken inconjunction with the accompanying drawings in which:

FIG. 1 illustrates a module assembly which may also include componentsto render the module assembly a secure module assembly.

FIGS. 2 to 5 illustrate the formation of a flex circuit wherein

FIG. 2 illustrates a flat piece of the flex circuit before it isdeformed in a shape necessary to match that of the enclosure.

FIG. 3 illustrates the flat flex circuit in FIG. 2 has been deformed ina fixture assembly to form a recess.

FIG. 4 illustrates the flex circuit of FIG. 3 has been trimmed to removeexcess material.

FIG. 5 illustrates the trimmed flex circuit of FIG. 4 has been placedwithin the enclosure.

FIGS. 6 to 11 illustrate various views of a fixture assembly fordeforming the flex circuit wherein:

FIG. 6 illustrates an exploded view of the fixture assembly 30 fordeforming the flex circuit.

FIG. 7 illustrates a perspective view of the fixture assembly in itsfully assembled state.

FIGS. 8 to 11 illustrate the various components of the fixture assembly,except for the punch, as the fixture assembly is being assembledwherein:

FIG. 8 illustrates a base plate having a recess.

FIG. 9 illustrates a flat flex circuit placed on the base plate.

FIG. 10 illustrates the wrinkle reducer placed on the flat flex circuit.

FIG. 11 illustrates the stiffening block placed on the wrinkle reducer.

FIG. 12 illustrates the operation of the punch where the maximumdownward movement of the punch is shown.

FIG. 13 illustrates a plan view of the wrinkle reducer.

FIG. 14 illustrates in greater detail the portion of the fixtureassembly outlined by the dotted line box in FIG. 12.

FIG. 15 illustrates a process for forming a flex circuit using a fixtureassembly.

FIG. 16 illustrates a process in which the deformed flex circuitfabricated by the process in FIG. 15 may be further processed.

DETAILED DESCRIPTION

Referring to the drawings in more detail, and particularly referring toFIG. 1, there is illustrated a module assembly 10 which may also includecomponents to render the module assembly 10 a secure module assembly.

The module assembly 10 includes a circuit board 12, such as a printedcircuit board 12, on which may be mounted electronic components,generally indicated by 14. The particular electronic components 14 areunimportant to the present exemplary embodiments but may include, forthe purpose of illustration and not limitation, semiconductor devices,resistors, capacitors and input/output devices. Enclosing the electroniccomponents 14 may be a cover 16 which may be made from aluminum orstainless steel, for example.

Below the circuit board 12 may be a tamper-resistant enclosure 18. Theenclosure may be made from a metal such as aluminum or stainless steel.The tamper-resistant enclosure 18 may have feet 17 which may be used tobond the tamper-resistant enclosure 18 to the circuit board 12. The bond(not shown) may be, for example, an adhesive. Within the enclosure 18 isa flex circuit 20 which is formed so as to match the profile of theenclosure 18 and essentially lines the interior surface of the enclosure18. The flex circuit 20 may contain several layers and on one or morelayers, there may be security circuits, such as serpentine circuits,that may be connected to tamper detection circuits. The tamper detectioncircuits may erase encryption keys if an intrusion is detected, forexample, if one or more of the tamper detection circuits is broken ordisturbed. Located on the flex circuit 20 may be electronic components,generally indicated by 22. Again these electronic components 22 mayinclude, for the purpose of illustration and not limitation,semiconductor devices, resistors, capacitors and input/output devices.

In the module assembly 10, the enclosure 18 is a low profile designwhich requires in the exemplary embodiments to form the flex circuit 20to match the profile of the enclosure 18. For the purpose ofillustration and not limitation, the exterior height of the enclosure 18may be 0.5 to 2 mm as measured from the point of attachment to thecircuit board 12.

A flex circuit is a flexible circuit board that has been made by layingdown a layer of insulating material followed by layers of additionalinsulating material having metallic circuitry, for example, copper,formed on the insulating material. Topping the flex circuit may beanother layer of insulating material. The various layers may be bondedtogether by a nonconductive adhesive. For purposes of illustration, theinsulating material may be Kapton® polyimide manufactured by E. I. duPont de Nemours.

Referring now to FIGS. 2 to 5, there is illustrated the formation of theflex circuit 20. In FIG. 2, there is illustrated a flat piece of theflex circuit 20 before it is deformed in a shape to match that of theenclosure 18. The flex circuit 20 is completed except for deforming intothe desired shape. That is, the multiple layers of circuitry have beenformed and bonded together to form the flex circuit 20. Tooling holesare shown along the perimeter of the flex circuit 20.

For the purpose of illustration and not limitation, the flex circuit 20may have four layers of copper traces with each of the copper tracesbeing 4 mils wide and a separation of 4 mils between copper traces. Theinsulating material may be Kapton® polyimide.

Referring now to FIG. 3, the flat flex circuit 20 in FIG. 2 has beendeformed in a fixture assembly (to be described hereafter) to form arecess 24. FIG. 3 is an enlarged view of FIG. 2 so that the toolingholes on the perimeter of the flex circuit 20 are not shown. Withinrecess 24 is located the electronic circuitry 26 for the flex circuit20. The electronic circuitry 26 may include security circuits, such asserpentine circuits, that may be connected to tamper detection circuits.The electronic circuitry 26 is covered by a protective layer (not shown)to protect the electronic circuitry 26 from damage. The protective layerhas no security value other than to protect the electronic circuitry 26from damage.

The original flat portion 28 of the flex circuit 20 is no longer neededafter the deformation step in FIG. 3. Most of the excess flat portion 28of the flex circuit 20 is then cut off. The flex circuit 20 without theexcess flat portion 28 is shown in FIG. 4.

Referring now to FIG. 5, the remaining portion of the flex circuit 20with the recess 24 is placed within the enclosure 18. FIG. 5 has beenslightly rotated to show a side portion of the enclosure 18. Beforeplacing the flex circuit 20 within the enclosure 18, adhesive (notshown) may be applied to the flex circuit 20 and/or the interior of theenclosure 18 so that the flex circuit 20 may be bonded to the enclosure18. Any electronic components 22 (FIG. 1) may be joined to the flexcircuit 20 and then the enclosure 18 with the flex circuit 20 may bebonded, such as by an adhesive, to the circuit board 12.

FIG. 6 illustrates an exploded view of a fixture assembly 30 fordeforming the flex circuit 20. FIG. 7 illustrates a perspective view ofthe fixture assembly 30 in its fully assembled state. FIGS. 8 to 11illustrate the various components of the fixture assembly 30, except forthe punch, as the fixture assembly 30 is being assembled.

In the following discussion, FIGS. 6 to 11 should be referred to.

The fixture assembly 30 includes a base plate 32 having alignment pins34 for aligning the other components of the fixture assembly 30 andheaters 36 for heating the base plate 32. The base plate 32 may alsoinclude a recess 38 into which the flex circuit 20 will be deformed.

On top of the base plate 32 may be placed a flat flex circuit 20. Thealignment pins 34 may be used to align the flex circuit 20 to thecorrect position on the base plate 32. The portion 40 of the flexcircuit 20 within lines 42 is the part of the flex circuit 20 that maybe deformed to form a recess 24 (FIGS. 2 to 5). The portion 40 is alsowhere the electronic circuitry 26 has been previously formed.

A wrinkle reducer 44 may be placed on the flex circuit 20. The alignmentpins 34 may be used to align the wrinkle reducer 44 to the correctposition on the base plate 32 and also correctly align the wrinklereducer 44 with respect to the flex circuit 20. The wrinkle reducer 44may include a cut-out portion 46 which may correspond to the shape ofthe recess 38 in the base plate 32. The wrinkle reducer 44 limits thewrinkles that may be formed during the deformation of the flex circuit20. These wrinkles are usually formed in the corners.

A stiffening block 48 may be placed on the wrinkle reducer 44 using thealignment pins 34 to assure correct alignment in the fixture assembly30. The stiffening block 48 may include a cut-out portion 52 which maycorrespond to the shape of the recess 38 in the base plate 32. In oneexemplary embodiment, the cut-out portion 52 may be larger than thecut-out portion 46 in the wrinkle reducer 44 so that the wrinkle reducer44 may be partly exposed through the cut-out portion 52

The various components of the fixture assembly 30 and the flex circuit20 may be rigidly secured by, for example, screws 50.

Further included with the fixture assembly 30 may be a punch 54 whichfits within the cut-out portion 52 in the stiffening block 48 and isaligned to the fixture assembly 30 by the alignment pins 34. The punch54 may freely move up and down within the fixture assembly 30. Duringthe downward movement of the punch 54, the body 56 of the punch 54 maymove through the cut-out portion 52 in the stiffening block 48 and thecut-out portion 46 in the wrinkle reducer 44 to contact the flex circuit20. Further downward movement of the punch 54 may cause the deformationof the flex circuit 20 into the recess 38 of the base plate 32 to formrecess 24 in the flex circuit 20. Downward movement of the punch 54 islimited by punch extensions 58 which extend from the punch body 56. Atthe furthest downward movement of the punch 54, the punch extensions 58hit against the stiffening block 48 to limit any further downwardmovement of the punch 54. Pressure may be applied to the punch 54 by,for example, an Instron® testing machine. After the flex circuit 20 hasbeen deformed, the pressure may be removed from the punch 54 and thepunch 54 raised.

The punch 54 may additionally have cavities 60 for running a heatingfluid to heat up the punch and a cooling fluid to cool down the punch.

All of the base plate 32, wrinkle reducer 44, stiffening block 48 andpunch 54 may be made of metal such as aluminum or stainless steel.

Referring now to FIG. 12, the operation of the punch of the fixtureassembly 30 will be described in more detail where FIG. 12 illustratesthe maximum downward movement of the punch 54.

The punch body 56 pushes down against the flex circuit 20 to cause theflex circuit 20 to be deformed into the recess 38 in the base plate 32.In its full downward travel the punch extensions 58, one of which isshown in FIG. 12, hits against the spacer 59 which acts as a hard stopto prevent further movement of the punch 54 downwardly. The spacer 59 isused so that the depth that the punch 54 goes down may be calibrated.

In one exemplary embodiment, the punch body 56 need not hit the bottomof the recess 38 but only extend far enough into the recess 38 tosufficiently deform the flex circuit 20. In another exemplaryembodiment, the punch body 56 may have a notch 62 with enough clearanceto avoid the vertical surfaces of the wrinkle reducer 44 and the recess38 of the base plate 32. The spacer 59 prevents the horizontal surfaceof the notch 62 from impinging on the wrinkle reducer 44.

Referring now to FIG. 13 there is shown the wrinkle reducer 44 havingcut-out portion 46 and FIG. 14 illustrates the portion of the fixtureassembly 30 outlined by dotted line box 64 in FIG. 12 in greater detail.Referring first to FIG. 13, for the purpose of illustration and notlimitation, a line 66 has been shown on the surface of the wrinklereducer 44. The outer portion 68 of the wrinkle reducer 44, to theoutside of line 66, is the part that clamps the flex circuit 20 to thebase plate 32. The clamping force is provided by the stiffening block48. The inner portion 70 of the wrinkle reducer 44, to the inside ofline 66, does not actually clamp the flex circuit 20 but rather allowsthe flex circuit 20 to stretch during deformation of the flex circuit20. The relative proportions of the clamp zone and stretch zone may beempirically determined.

Referring now to FIG. 14, it can be seen that the flex circuit 20 isfirmly clamped between the wrinkle reducer 44 and the base plate 32 inthe clamp zone. However, in the stretch zone there is an approximate 3mil space 72 between the flex circuit 20 and the wrinkle reducer 44 sothat any wrinkles that form in the flex circuit 20 during thedeformation process do not have room to grow. Any wrinkles that form areusually formed near the corners. The 3 mil space 72 keeps the amplitudethat is the vertical height, of the wrinkles to a minimum.

Referring now to FIG. 15, there is illustrated a process for forming aflex circuit using a fixture assembly, such as the fixture assembly inFIGS. 6 to 11, which includes a base plate having a recess, a wrinklereducer, a stiffening block and a punch.

The process includes heating the fixture assembly 30 to a predeterminedtemperature, box 74. In one exemplary embodiment, the heatingtemperature may be about 130° C. The upper limit may be chosen to avoidmaterial damage to the flex circuit 20.

The fixture assembly 30 may be heated by heating the base plate 32 withheaters 36 and running a hot fluid through one of the cavities 60 in thepunch 54. The temperature may be monitored by thermocouples (not shown)in multiple places in the fixture assembly 30. It may be desirable tolet the fixture assembly 30 sit at the predetermined temperature for aperiod of time, for example 5 minutes, to assure that the fixtureassembly 30 has been evenly heated.

In one exemplary embodiment, the flex circuit 20 may be clamped betweenthe wrinkle reducer 44 and the base plate 32, box 76.

A force may be applied to the punch 54 causing the punch 54 to contactthe flex circuit 20 and deform the flex circuit 20 into the recess 38 inthe base plate 32, box 78. It may be desirable to move the punch 54downwardly at a constant rate until the punch 54 reaches its maximumlimit of travel. In one exemplary embodiment, the maximum limit oftravel may be reached when the punch extensions 58 hit against thespacer 59 or, if there is no spacer 59, then stiffening block 48.

The punch 54 may be held in contact with the deformed flex circuit 20for a predetermined amount of time, for example 5 minutes, while thefixture assembly is heated at the predetermined temperature, box 80.

The fixture assembly 30 may be cooled to a second predeterminedtemperature with the punch 54 in contact with the deformed flex circuit20, box 82. The second predetermined temperature may be, for example,100° C. and no higher. The 100° C. temperature is preferred to avoidforming steam during cooling of the fixture assembly 30.

To cool the fixture assembly 30, air may be used.

After the fixture assembly 30 has reached the second predeterminedtemperature, the force of the punch 54 may be removed, box 84.

The flex circuit 30 may be removed from the fixture assembly 30, box 88.In one exemplary embodiment, fixture assembly 30 may be cooled to roomtemperature by water prior to removal of the flex circuit 20 from thefixture assembly 30, box 86. The flex circuit removed from the fixtureassembly 30 may have a recessed portion 24 resulting from deforming theflex circuit 20 into the recess 38 in the base plate 32 and a flatportion 28 surrounding the recessed portion 24.

Referring now to FIG. 16, in one exemplary embodiment, the deformed flexcircuit 20 deformed by the process in FIG. 15 may be further processed.The flat portion 28 surrounding the recessed portion 24 may be trimmedsuch as by cutting most of the excess off the flex circuit 30. For thepurpose of illustration and not limitation, there may be maintained aflat border of approximate width 2 mm (millimeters) surrounding therecessed portion 24, box 90. However, the width of the flat borderremaining depends on the application. Thus, there may be more than 2 mmleft as the flat border or there may be no flat border at all.

The recessed portion 24 of the flex circuit 20 may be bonded, forexample by an adhesive (not shown), to a metallic enclosure 18, box 92.

The flex circuit 30 and metallic enclosure 18 may be bonded, for exampleby an adhesive (not shown), to a circuit board 12, box 94.

It will be apparent to those skilled in the art having regard to thisdisclosure that other modifications of the exemplary embodiments beyondthose embodiments specifically described here may be made withoutdeparting from the spirit of the invention. Accordingly, suchmodifications are considered within the scope of the invention aslimited solely by the appended claims.

What is claimed is:
 1. A process for deforming a flex circuit using afixture assembly comprising a base plate having a recess, a wrinklereducer plate, a stiffening block and a punch, the process comprising:heating the fixture assembly to a predetermined temperature; applying aforce to the punch causing the punch to contact the flex circuit, anddeforming the flex circuit into the recess in the base plate; holdingthe punch in contact with the flex circuit for a predetermined amount oftime while heating the fixture assembly at the predeterminedtemperature; cooling the fixture assembly to a second predeterminedtemperature with the punch in contact with the flex circuit; subsequentto the fixture assembly reaching the second predetermined temperature,removing the force of the punch; and removing the flex circuit from thefixture assembly, the flex circuit having a recessed portion resultingfrom deforming the flex circuit into the recess in the base plate and aflat portion surrounding the recessed portion.
 2. The process of claim 1further comprising cooling the fixture assembly to room temperatureprior to removing the flex circuit from the fixture assembly.
 3. Theprocess of claim 1 further comprising clamping the flex circuit betweenthe wrinkle reducer and the base plate.
 4. The process of claim 3wherein the wrinkle reducer has a cut-out portion aligned with therecess wherein the wrinkle reducer has a clamp zone at an edge of thewrinkle reducer in which the flex circuit is clamped and a stretch zonebetween the clamp zone and the first cut-out portion in which the flexcircuit is not clamped when the flex circuit is placed between the baseplate and the wrinkle reducer.
 5. The process of claim 4 wherein thewrinkle reducer has a first thickness in the clamp zone and a secondthickness in the stretch zone wherein the second thickness is less thanthe first thickness so as to provide a gap between the wrinkle reducerand the flex circuit.
 6. The process of claim 1 further comprising:trimming the flat portion; and bonding the recessed portion to ametallic enclosure.
 7. The process of claim 6 further comprising bondingthe flex circuit and metallic enclosure to a circuit board.
 8. Theprocess of claim 1 wherein the flex circuit having one or more tamperdetection circuits.
 9. A process for deforming a flex circuit using afixture assembly comprising a base plate having a recess, a wrinklereducer plate, a stiffening block and a punch, the process comprising:clamping the flex circuit between the wrinkle reducer and the base platewherein the wrinkle reducer has a cut-out portion aligned with therecess and wherein the wrinkle reducer has a clamp zone at an edge ofthe wrinkle reducer in which the flex circuit is clamped and a stretchzone between the clamp zone and the first cut-out portion in which theflex circuit is not clamped when the flex circuit is placed between thebase plate and the wrinkle reducer; heating the fixture assembly to apredetermined temperature; applying a force to the punch causing thepunch to contact the flex circuit, and deforming the flex circuit intothe recess in the base plate; holding the punch in contact with the flexcircuit for a predetermined amount of time while heating the fixtureassembly at the predetermined temperature; cooling the fixture assemblyto a second predetermined temperature with the punch in contact with theflex circuit; subsequent to the fixture assembly reaching the secondpredetermined temperature, removing the force of the punch; and removingthe flex circuit from the fixture assembly, the flex circuit having arecessed portion resulting from deforming the flex circuit into therecess in the base plate and a flat portion surrounding the recessedportion.
 10. The process of claim 9 wherein in the step of applying aforce, stopping the punch before the punch and flex circuit hits thebottom of the recess.
 11. The process of claim 9 wherein in the step ofapplying a force, applying the force to cause the punch to movedownwardly at a constant rate until the punch reaches its maximum limitof travel.
 12. The process of claim 9 further comprising cooling thefixture assembly to room temperature prior to removing the flex circuitfrom the fixture assembly.
 13. The process of claim 9 wherein thewrinkle reducer has a first thickness in the clamp zone and a secondthickness in the stretch zone wherein the second thickness is less thanthe first thickness so as to provide a gap between the wrinkle reducerand the flex circuit.
 14. The process of claim 9 further comprising:trimming the flat portion; and bonding the recessed portion to ametallic enclosure.
 15. The process of claim 14 further comprisingbonding the flex circuit and metallic enclosure to a circuit board. 16.The process of claim 9 wherein the flex circuit having one or moretamper detection circuits.